Blue Nile is not the only pure-play online retailer to appreciate this fact. Warby Parker, a well-known online purveyor of eyeglasses to Internet-savvy hipsters, began branching out into the physical world a couple of years ago. It has storefronts in nine cities, as well as showrooms in some boutiques.

In Seattle, MiiR, an online venture that sells bikes, bike bottles and growlers and shares the profits with poverty-fighting efforts, opened a big store in Fremont last month.

Blue Nile’s store opened Friday at the Roosevelt Field Mall on Long Island, the same shopping center where the local Nordstrom housed its temporary store.

There will be only three cases of product, showcasing ring styles. When customers come, they’re led to a bar (“our version of the Genius bar,” says CEO Harvey Kanter) where the employees walk them through the company’s 250,000 stones on an oversized tablet. As part of Blue Nile’s no-pressure mantra, the employees are not on commission.

All sales are still made online — but the goods can be ordered and picked up at the store, which will also offer free cleanings and repair.

Kanter says that Blue Nile decided to place the store at the Roosevelt Field mall because of the successful trial there in Nordstrom, but also because Simon Property Group, the mall’s owner, is aggressively courting pure-play e-commerce retailers to establish physical outlets.

For Blue Nile, which made its name selling discounted diamonds over the Internet on an information-packed website, the store is a way to increase its knowledge about the brick-and-mortar world.

For instance, from its Nordstrom experience the company learned that customers who purchased items after visiting spent an average of 8 percent more than the average web-only shopper, but it doesn’t know exactly why.

Kanter says it might be simply that the clientele in a Nordstrom is more moneyed than the average, but that’s just a theory.

“This is about learning and charting a road map,” Kanter says. But he is optimistic. “We certainly didn’t open the store with the vision it is going to be the only one.”